Emissions from automobiles are a major cause of air pollution. It is therefore of extreme importance to develop a technique for removing noxious components in automobile emissions.
It is particularly important and urgently required to develop a technique for removing particulates contained in diesel engine emissions, which are mainly NOx and carbon.
To remove such noxious components in exhaust gas, it has been proposed to provide a particulate trap in an exhaust gas line to trap and remove any noxious components. Other conventional exhaust gas purifying means include various improvements in the engine itself such as exhaust gas recirculation (EGR) systems and improved fuel injection systems. But none of them are decisive solutions.
As of today, it is considered more promising to treat exhaust gas with a trap. Rigorous efforts are now being made to improve such traps.
A particulate trap comprises a case mounted in an exhaust gas line, and a filter element housed in the case. Exposed to high-temperature exhaust gas, the filter element has to be highly heat-resistant. Also, in order to efficiently trap particulates, the filter element should have as large a trapping area as possible. It is also necessary to prevent leakage of exhaust gas through its mounting portion.
Moreover, the filter element has to be vibration-resistant enough not to be damaged or broken due to vehicle vibration, simple in structure so as to be manufactured at low cost. It is also required that the energy cost for regeneration of the filter element, i.e. for burning particulates trapped be as low as possible.
In order to meet all these requirements, various factors have to be taken into consideration, including the material and shape of the filter element, end sealing structure, and the structure for securing the filter element to the case.
Such particulate traps are disclosed e.g. in unexamined Japanese patent publications 6-146856, 6-264715, 5-222920, and 6-257422.
Particulates trapped have to be periodically removed for regeneration of the filter. Unexamined Japanese patent publications 5-22290, 6-146856, 6-264722 propose to burn particulates with an electric heater. These traps have a plurality of filters arranged in parallel. Some of these traps (such as one disclosed in unexamined Japanese patent publication 6-101449) have an exhaust gas purifier for regenerating the filter by burning while the engine is running without extremely reducing the heat efficiency of the heater (by controlling the flow rate of exhaust gas by means of valves) and while supplying enough oxygen for complete combustion of soot (particulates).
But in the case of a trap of the type that controls the amount of emissions, a delicate touch is required for such control. Also, a device for controlling the amount of emissions has to be resistant to heat from exhaust gas. Naturally, the entire trap tends to be complicated in structure and costly.
There is also known a relatively simple exhaust gas purifier of a type in which particulates are burned for regeneration of the filter while the engine is at a stop. Some of the devices of this type resort to an external air source or an electric fan for supplying oxygen necessary to burn particulates trapped during the regeneration step. More recent devices have a filter case having a shape that makes it possible to supply oxygen by natural convection, thereby eliminating the need for an external or auxiliary device for supplying oxygen.
Any of the particulate traps disclosed in the above-listed publications uses cylindrical filters. The trap disclosed in the publication 6-146856 has insulating members provided at either end of the cylindrical filters with one of the insulators sandwiched between the cylindrical filters and the case and the other pressed against the filter end by a screw-tightened presser to fix the filters and seal the filter ends. In this arrangement, the insulators tend to be damaged by heat of exhaust gas and vehicle vibration. Also, gap may be formed between sealing surfaces due to differences in thermal expansion between the filter and the case. It is thus difficult to completely prevent the leakage of exhaust gas.
The trap disclosed in the publication 6-264715 has cylindrical filters having one end thereof closed by fitting a protrusion of a support plate provided in the case, and the other end fixed to the case by a bracket. In this arrangement, due to difference in thermal expansion between the filter and the support plate, gap may be formed in the closed portion through which exhaust gas can leak. Such a gap permits displacement of the filters. The support portions of the filters are thus likely to be damaged by vibration.
The trap disclosed in the publication 5-222920 has a cylindrical filter having a closed bottom and thus is free of problem of reduced sealing at one end of the filter. But such metal filters are difficult to mass-produce and thus costly.
The trap proposed by the applicant of this invention in the publication 6-257422 has a filter element comprising at least two cylindrical filters having different diameters from each other and concentrically fitted together. This trap has a large particulate trapping area. Particulates trapped can be efficiently burned with a heater provided between the two cylindrical filters. Exhaust gas incoming and outgoing spaces are formed by alternately closing the openings at either end with end plates. Such a filter element can be manufactured easily, though it is equivalent in function to an integrally formed cylindrical filter having a closed bottom.
One problem with this trap is that it is difficult to fix e.g. the end plates in position. Brazing and welding are possible ways to fix such end plates and brackets. But brazed portions cannot withstand high temperature when particulates are burned.
If a three-dimensionally reticulated porous metal (known as "expanded metal") is used as a filter material, which is high in porosity and thus advantageous in reducing pressure loss, welding is impossible with an ordinary welding method such as electric or gas welding.
Even though there is known a special welding method for welding such filter members, the thus welded porous filter members could not stably support each other or other members. The trap formed will thus be low in vibration resistance.
The best way to dispose of particulates trapped is to burn them because by burning particulates, the trap can be used repeatedly. Use of a light oil burner is being considered as a possible means to burn particulates trapped. But now an electric heater is considered to be the most promising means because it is safe and easy to control. Thus, electric heaters are used in many particulate traps, as disclosed in unexamined Japanese patent publications 5-22290, 6-257422, and 6-264722.
In order to efficiently heat the filter, an electric heater having two opposite surfaces may be inserted in a longitudinally recessed exhaust gas incoming space to simultaneously heat two opposite filtering surfaces, as disclosed in unexamined Japanese patent publication 6-257422.
In order to stably support such a heater, the heater may be screwed to a support bar inserted in the exhaust gas-incoming space (defined inside the inner cylinder) together with the heater, as disclosed in the same publication.
Simply by inserting the electric heater between the opposite filtering surfaces, however, it is impossible to stably support the heater. If the heater is thermally expanded and deformed while being energized to such an extent that it contacts the filter, electric leakage may occur. When such a trap is mounted on a vehicle, the heater may come into contact with the filter due to vibration, causing electric leakage. These have been major obstacles to the development of practically usable particulate traps. If electric leakage occurs, it is impossible to burn particulates trapped any more. Since particulates cannot be removed, the exhaust gas resistance will grow gradually with use. In the worst case, the exhaust gas resistance may increase to such a degree that the vehicle engine stops.
In the arrangement in which the heater is screwed to a support bar, a complex insulating structure is needed. Such a structure, made up of a large number of parts, pushes up the cost of the trap. Further, heater-fixing members including the support bar, setscrews, insulators add to the heat capacity of the filter element. Thus, large part of the heat produced by the heater will escape to these fixing members and then to the outside, increasing the heater capacity. Also, regeneration tends to be insufficient around the screwed portions.
FIG. 17 shows a mechanism for supplying oxygen by natural convection to a particulate trap of a type in which the filter is regenerated only while the engine is not running.
The particulate trap shown has a filter element 3 comprising cylindrical filters having different diameters, i.e. an inner cylindrical filter 4 and an outer cylindrical filter 5 concentrically provided around the inner filter 4. End plates 6 are provided to close the opening of the inner cylindrical filter 4 at the exhaust gas incoming end, and the opening between the inner and outer cylindrical filters at the exhaust gas outgoing end. Exhaust gas flows into the space between the inner and outer cylindrical filters 4 and 5 through its inlet, and passes through the filtering portions of the respective cylindrical filters into exhaust gas outgoing spaces defined inside the inner cylindrical filter 4 and outside the outer cylindrical filter 5.
This particulate trap, having a multiple-cylinder type filter element, has a large filtering area. The electric heater 8 inserted between the opposite filtering surfaces as shown can heat the entire filtering areas uniformly and efficiently. It is thus possible to reduce the size of the entire trap and the power consumed by the heater.
In the arrangement shown, in which a case 2 for a conventional single-cylinder type filter having an air inlet 11 through which air is introduced by natural convection and an air outlet 12 is used for a multiple-cylinder type filter, ambient air introduced into the case during filter regeneration cannot be distributed sufficiently into the cylindrical filters. Since sufficient air is not supplied into the filters, particulates cannot be burned smoothly. Regeration thus takes a long time, increasing the power consumption for regeneration. In an extreme case, the filter may not be sufficiently regenerated. An object of the present invention is to provide a particulate trap having none of these problems.